This invention relates to the heat disinfection of contact lenses.
Contact lenses collect microorganisms which, if permitted to grow, can infect the eye of the wearer. Consequently, it is necessary to clean and disinfect the lens, preferably daily, to kill microorganisms on the lens. This necessity is especially pronounced with soft contact lenses, which are made of material that is permeable to water and into which microorganisms can penetrate.
One known method of disinfecting a contact lens comprises immersing the lens in an aqueous medium, typically a buffered saline solution, and heating the immersed lens at a moderately elevated temperature (e.g. 80.degree. C.-95.degree. C.) for enough time, typically at least 10 minutes, to kill any microorganisms that may be present in or on the lens. The heating is carried out in a small device whose basic elements include a chamber for holding the case that contains the immersed lens, and a heating element that heats the chamber.
It is highly desirable to maintain a relatively uniform temperature throughout the chamber, so that even if the location of the lens-carrying case shifts within the chamber the lenses will be heated sufficiently and will not be heated to too high a temperature. This objective becomes more pronounced as the shapes and sizes of lens-carrying cases proliferate; since the chamber should be designed to be able to accommodate as many such shapes and sizes as possible, any lens case smaller than the largest case that can fit into the chamber will be capable of moving about within the chamber if the unit is bumped. It becomes necessary to heat the chamber sufficiently hot that the lenses are disinfected regardless of where they are located in the chamber. If the actual heating element is smaller than the chamber, this can lead to the area of the chamber closest to the element being so hot that the lens can be damaged.
Prior art units have dealt with this problem by heating the chamber through a heating block which absorbs a significant quantity of heat from an electrical-resistance heating element and releases the absorbed heat gradually to the chamber. The operation is controlled by a thermostat which shuts off the heating element when the block reaches a predetermined temperature, i.e. has absorbed a predetermined quantity of heat, substantially before the end of the disinfecting cycle. This type of system has several drawbacks. The heating element must be large enough as to heating capacity and as to physical size to heat the block sufficiently in an acceptable short length of time while maintaining a relatively uniform temperature across the top of the block. In addition, the block must be relatively large and dense, which fact adds to the weight of the unit and increases the expense of making and shipping the unit. Discontinuing the application of heat before the disinfection cycle ends requires that the lenses be heated to a temperature at least as high as the preferred disinfection temperature range, at which point the element is shut off; this exposes the lenses to temperatures that could be unacceptably close to temperatures at which the lenses would be damaged. Since most of the disinfection is performed while heat is dissipated from the block without additional heat input from the element, operation is vulnerable to loss of heat from the block to unexpectedly cool ambient atmosphere, to premature opening of the unit by the operator, and the like. The chamber is actually in the optimum temperature range for only a brief period; otherwise the chamber is cooler.
Illustrative of such units are those described in U.S. Pat. No. 4,044,226 and U.S. Pat. No. 4,307,289. Both patents disclose heat disinfection units that include a heating block having relatively high heat absorptive ability, and a thermostatic switch that shuts off the application of heat from the heating element to the block a substantial length of time before the disinfection cycle is complete. U.S. Pat. No. 4,044,226 actually discourages the use of a timing circuit. In addition, U.S. Pat. No. 3,983,362 describes a similar principle of operation, and even discourages using a "heat member" which gives up absorbed heat rapidly, on the ground that the interior of the carrying case cannot be maintained at an acceptable temperature.
The present invention avoids these and other drawbacks and presents the advantages discussed below, by employing components (e.g. a timing circuit, and a thin plate rather than a heating block) which the prior art teaches away from.